Hot water tanks with a small capacity, particularly for household purposes, have been provided heretofor with a passive anticorrosive layer (usually of enamel), as well as with a donor anode of magnesium. The magnesium anode is usually screwed into the tank from the top through a nipple, while the electric heating system is arranged above a flange in the lower part. It was found in practice that these hot water tanks are destroyed by corrosion after a few years, particularly in the lower region receiving the heating element.
Thorough investigations by means of potential- and current measurements have shown that the lower third will no longer be protected cathodically in such hot water tanks when the heating flange is not insulated from the tank. This insulation is not usually provided for electrotechnical reasons. The use of magnesium donor anodes has other disadvantages, namely a magnesium anode has considerable natural corrosion so that it is also frequently completely destroyed or used up after a few years. Furthermore, a considerable amount of hydrogen gas is formed on a magnesium anode which combines with oxygen to form a dangerous oxyhydrogen mixture.
In larger tanks, it has already been suggested to use, instead of magnesium donor anodes, electrodes supplied with external current. This does not lead to a satisfactory result either, because the external current electrodes used in the past for larger hot water tanks were always set to a single current. Accordingly, when the conditions changed in the tank, e.g., when an additional defective spot appeared in the passive anticorrosive layer, the correct potential was no longer set on the tank wall. Furthermore, the present external current electrodes do not provide a satisfactory solution of the special corrosion hazard in the area of the heating element of a water tank equipped with such a heating element. The damage in this region is caused by the surfaces acting as a cathodic region on which oxygen is reduced so that they can act as oxidants, and any defective spots in the passive anticorrosive layer appearing in this region are particularly subject to corrosion.
In order to achieve a long-term improvement of the corrosion protection in these tanks, it is suggested in copending U.S. patent application Ser. No. 764,903 filed Feb. 1, 1977 and now abandoned, to design the electrodes so that they can be controlled to an optimum potential by means of a known potentiostat and a similarly known reference electrode. As indicated in the aforementioned copending application, potentiostats together with suitably spaced anodes and reference electrodes, have been used in the protection of ships hulls and pipes containing flowing water. Prior to the copending U.S. patent application, it was believed that while the approach was satisfactory in systems wherein gaseous by-products would freely pass into the atmosphere, the hydrogen and oxygen produced electrochemically by the current flow would, in closed systems such as hot water tanks, collect as a dangerous explosive mixture. It was discovered, however, that by maintaining the tank wall at a negative potential, and by the automatic current regulation provided by the potentiostat, dangerous accumulations of the oxyhydrogen mixture were avoided. In this application, at least one external current anode is arranged in the proximity of the heating element, possibly centrally and equally far from all wall surfaces. The reference electrode, however, is to be as close as possible to the tank wall, so that the reference electrode and the external current anode are arranged separately according to this copending U.S. Patent application and are connected with the potentiostat by separate lines.
Because of this arrangement, separate holders must be provided for the reference electrode and the external current anode, and correspondingly, separate openings or inserts must be provided in the tank wall. In this assembly, connections must be provided between the potentiostat and the reference electrode, between the potentiostat and the external current anode, and between the potentiostat and the surface or tank wall. The manufacture of such tank requires a great expenditure of energy and time by skilled workers.